An emerging area in the field of seismology is the area of borehole seismology. In traditional seismology, both a source and sensors have been either located at the surface, or the sensors (“receivers”) have been located downhole while the source has been located on the surface. In borehole seismology, the source is placed in a borehole while the receivers can be either on the surface, or preferably in a borehole as well. This later mode is known as “cross-well seismology.” Borehole seismology is particularly useful in determining the condition of an existing reservoir, following the history of a producing reservoir, and exploring potential new reservoirs. Borehole seismology also makes it possible to routinely record shear waves, which allows for mapping lithology of oil and gas reservoirs.
A limiting factor in borehole seismology has been the lack of receiver arrays for boreholes which provide the dense spatial sampling required to make use of the high seismic frequencies made possible by the consolidated geologic formation. Shear (“S”) waves, for example, have only half the wave length of compressional (“P”) waves, further increasing the need for dense spatial sampling. The recording of compressional waves as well as polarized shear waves makes it possible to map the mechanical properties of oil and gas reservoirs, as well as map and distinguish between different fluids and the effect of lithology. This information can also be used to map differential field stresses, which is the primary source for differential permeability in a reservoir. Further, high signal to noise ratios, as well as a dense spatial sampling, allow for direct use of attenuation of compressional and shear waves for characterization of oil and gas reservoirs. This combination of seismic measurements allows much more information to be extracted about the true nature of oil and gas reservoirs.
In order to record and collect this required volume of measurements from borehole seismology, what is needed is a seismic receiver array which can be deployed within a borehole and which has the capability of detecting both compressional and shear waves, as well as transmitting this information from the borehole to the surface where it can be further collected and/or processed. However, the borehole environment makes it difficult to record useful seismic data for borehole seismology. Merely lowering an array of hydrophones into a borehole is typically insufficient to record the data necessary for useful borehole seismology. Hydrophones are susceptible to recording energy from tube wave noise, which can obscure useful seismic signals. Further, in a gas-filled well hydrophones are useless, as the gaseous fluid in the borehole does not conduct the energy from the borehole to the hydrophone.
Therefore, what is needed is a receiver which can be used for borehole seismology. More particularly, what is needed is a receiver array which can be deployed within a borehole and which will record shear and compressional waves useful in characterizing the reservoir, as well as transmit the received data to a surface location where it can be utilized.